Resonant tunneling diodes as sources for millimeter andsubmillimeter wavelengths

High quality resonant tunneling diodes have been fabricated and tested as sources for millimeter and submillimeter wavelengths. The devices have shown excellent I-V characteristics with peak-to-valley current ratios as high as 6:1 and current densities in the range of 50-150 kA/cm² at 300 K. Used as local oscillators, the diodes are capable of state of the art output power delivered by AlGaAs-based tunneling devices. As harmonic multipliers, a frequency of 320 GHz has been achieved by quintupling the fundamental oscillation of a klystron source     

Al0.3Ga0.7As-GaAs microwave resonant tunneling oscillator

We describe the design of a microwave oscillator using resonant tunneling diodes. The devices are fabricated from Al_0.3Ga_0.7As-GaAs double barrier hetero-structures grown by molecular beam epitaxy. Design criteria improving current drivability are established from a theoretical study of tunneling transmission probabilities. Very high peak current densities up to 3.10⁴ A/cm², favorable for high frequency operation as an oscillator, have been achieved experimentally. The devices exhibit stable oscillations at liquid nitrogen temperature and at room temperature when the tunnel diode oscillator is constructed with a stabilizing network.     

Restoration of polarization in ferroelectric thin films studied with the model of trapping of carriers at interfacial states

A model based on trapping and releasing of free carriers in band gap states located at the interfaces between electrodes and the ferroelectric film is used to explain the restoration of polarization which is sometimes observed. Experimental data on two kinds of polarization restoration in metal/ferroelectric/metal thin film structures are reported. The increase of polarization, which may be observed at the early stage of fatigue test, is attributed to the release of carriers which are trapped in interfacial states at the virgin state, before fatigue test. On the other hand, the previously reported “self recovery effect” is attributed to a competition between the strength of a trapped carrier-ferroelectric dipole interaction and the applied voltage which acts on the dipole orientation.     

Compositionally graded Pb(Zr,Ti)O3 thin films with different crystallographic orientations

Compositionally graded ferroelectric PbZr_xTi_1−xO₃ (PZT) films were deposited using a sputtering method and crystallized in situ at 500 °C. The films showed purely (100) or (111) crystallographic orientation when grown on Si/SiO₂/TiO₂/Pt substrates, while they exhibited c-axis epitaxial microstructure when prepared on MgO/Pt substrates. Their crystallographic orientation was controlled owing to a thin TiO_x layer sputtered on substrates prior to PZT deposition. Analysis performed by Auger depth profile clearly confirmed the variation of composition in the films. Coercive fields from 80 kV/cm to 200 kV/cm and remnant polarization as large as 45 μC/cm² were obtained. However, no typical offset was observed on hysteresis loops, unlike previous works related to graded PZT films.